On Fri, 2021-01-29 at 09:23 +0100, Michal Hocko wrote:

On Thu 28-01-21 13:05:02, James Bottomley wrote:

quoted

Obviously the API choice could be revisited
but do you have anything to add over the previous discussion, or is
this just to get your access control?

Well, access control is certainly one thing which I still believe is
missing. But if there is a general agreement that the direct map
manipulation is not that critical then this will become much less of
a problem of course.

The secret memory is a scarce resource but it's not a facility that
should only be available to some users.

It all boils down whether secret memory is a scarce resource. With
the existing implementation it really is. It is effectivelly
repeating same design errors as hugetlb did. And look now, we have a
subtle and convoluted reservation code to track mmap requests and we
have a cgroup controller to, guess what, have at least some control
over distribution if the preallocated pool. See where am I coming
from?

I'm fairly sure rlimit is the correct way to control this.  The
subtlety in both rlimit and memcg tracking comes from deciding to
account under an existing category rather than having our own new one. 
People don't like new stuff in accounting because it requires
modifications to everything in userspace.  Accounting under and
existing limit keeps userspace the same but leads to endless arguments
about which limit it should be under.  It took us several patch set
iterations to get to a fragile consensus on this which you're now
disrupting for reasons you're not making clear.

If the secret memory is more in line with mlock without any imposed
limit (other than available memory) in the end then, sure, using the
same access control as mlock sounds reasonable. Btw. if this is
really just a more restrictive mlock then is there any reason to not
hook this into the existing mlock infrastructure (e.g.
MCL_EXCLUSIVE)? Implications would be that direct map would be
handled on instantiation/tear down paths, migration would deal with
the same (if possible). Other than that it would be mlock like.

In the very first patch set we proposed a mmap flag to do this.  Under
detailed probing it emerged that this suffers from several design
problems: the KVM people want VMM to be able to remove the secret
memory range from the process; there may be situations where sharing is
useful and some people want to be able to seal the operations.  All of
this ended up convincing everyone that a file descriptor based approach
was better than a mmap one.

James



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